This invention relates to a vehicle steering mechanism particularly applicable to fork lift truck vehicles. More specifically, this invention is directed to a steering guidance means associated with each steerable wheel of the vehicle which senses the vehicle position with respect to a lateral guide surface and mechanically transmits movement to control the turning of the associated wheel. The invention additionally includes a control means for removing the normal hydraulic power steering means from operation whereby the mechanical guidance mechanism is the sole controller of the vehicle steering.
Currently, a real effort is being made to conserve the floor space in storage warehouses by using narrower aisles and accommodating narrow aisle fork lift trucks. Ideally, the width of the narrow aisle should only be slightly greater than the maximum width of the fork lift truck vehicle which travels down the aisles loading and unloading material storage racks arrayed on opposite sides of the aisles. Since it is extremely difficult for an operator to negotiate such narrow aisles without grazing or otherwise hitting the storage racks, guide mechanisms have been developed to guide the vehicle in a precisely determined path within the narrow aisles between rows of storage racks. An additional benefit of these guide means is that they permit the operator to dispense with the function of steering the vehicle and allow him to devote his efforts to other tasks, such as finding the number of the appropriate load space.
Illustrative of one of these prior art guidance means is U.S. Pat. No. 3,850,111 to Hansen. With this patent device, a guide plate assembly mounted under the vehicle, such as a fork lift truck vehicle, has a plurality of rollers laterally extending therefrom on oposite sides of the vehicle. The rollers contact accommodating guide rails and the vehicle is slewed to follow a straight line path between the guide rails. This type of device is not entirely satisfactory, however. The principle reason it is not satisfactory is that there is no positive turning of the vehicle steering wheels. Rather, there is no positive control of the vehicle steering wheels exercised by the guide means. A certain amount of tire wear and vehicle shock is also encountered with this type of system since the unit basically slews or drags the vehicle sideways to maintain the correct aisle position.
However, with the prior art guide means, very strong floor-mounted guide rails are required because this brute force guidance approach results in such great forces that longitudinal members on the racks cannot be used. Such rails on the floor on each side of the aisle are expensive and interfere with the placing of loads directly on the floor so that a bottom shelf is required.
In order to overcome some of these disadvantages a guidance means has been developed to translate the motion of a guidance member contacting the lateral guide surface to the steerable wheels of a vehicle. Examples of this type of device are shown in U.S. Pat. No. 3,858,523 to Allsup, Jr., and U.S. Pat. No. 3,872,794 to Goode. However, while this system which uses the mechanical linkage for translating motion from the guidance member to the wheels of the vehicle is of some improvement, it is nonapplicable to a vehicle wherein the wheels thereof are normally steered by power means such as hydraulic cylinder means.